The impact of nontuberculous mycobacteria (NTM) on the natural course of cystic fibrosis (CF) lung disease is not known. The purposes of this study are to help define the prevalence of NTM in CF patients and to better delineate the clinical course of CF patients with NTM. Serial sputum cultures for NTM from CF patients will be obtained, and any patients found to have a new positive culture for NTM will be evaluated serially to define the extent of disease.